NASA Technical Reports Server (NTRS) 20000090565: The Ultimate Mountaintop: Astronomy Aboard Stratospheric Balloons

12 Pages·0.58 MB·English

by  NASA Technical Reports Server (NTRS)

Christopher Wanjck. Code 662
July .3. 2000
Title: "The Ultimate Nlountaintop -- Astronomy Aboard Stratospheric
Balloons"
Publication: for Sky & Telescope Magazine, September 2000
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The Ultimate Mountaintop
Astronomy Aboard Stratospheric Balloons
"...the air through which we look upon the stars is in a perpetual tremor...
The only remedy is a most serene and quiet air such as may perhaps be
found on the tops of the highest mountains above the grosser clouds." --
Optics, Sir Isaac Newton
Floating with the winds at heights higher than the Himalayas, the Swiss
psychiatrist-cum-adventurer Bertrand Piccard had a view of the night sky
that backyard astronomers can only dream about. His historical 19-day
non,stop balloon flight around the world beneath the silvery Breitling
Orbiter 3 last March had him hopscotching from one jet stream to the
next, far above the lights of the cities, far above the clouds.
"It was absolutely fabulous," said Piccard, the grandson of balloon pioneer
Auguste Piccard, the first man to enter the stratosphere. "We would shut
off all of our lights at night and watch the multitude of stars shining above
us. We were a little bubble floating around another bubble called earth,
which is itself floating in a vast universe."
Such is the lure of the balloon, silent and majestic envelopes of air that
lifted the Montgolfier brothers high above Paris 120 years before the
Wright brothers..wheeled their clumsy contraption onto the fields of Kitty
Hawk. Science recognized the potential of ballooning immediately. Na'l've
yet resolute atmospheric explorers in the t800s collected samples of air
as high up as their naked lungs could take them. Others mapped the
movement of the stars and planets during all-night voyages, seemingly
without a care to what village they would land in the next morning. In the
early 20th century, daylong balloonsonde missions brought the discovery
of the cosmic rays, gamma rays and stratosphere itself. In the pre-Apollo
,,'ears, balloons carried aeronauts to the brink of space.
Balloons, as playful as they seem, remain poised at the forefront of
science. Astronomers and physicists today envision floating, retrievable
observatories stationed four times higher than any passenger jet could
dare venture. Their goal is to develop balloons capable of staying afloat
for a hundred days or more at altitudes of nearly 40 kilometers. At that
height, less than a hundredth" of a percent of the atmosphere remains
above, and we will have reached the summit of the ultimate mountaintop.
It seems fitting, then, that Bertrand Piccard and his crewmate, Brian Jones,
have retired their tiny gondola to the National Air and Space Museum,
where it rests with the Wright Flyer, the Spirit of St. Louis and the Apollo
11 command module. For like these past triumphs of aviation, the
landmark balloon flight around the world is but a sign of fantastic things
to come.
To Push or Pull
On August 11, 1999, Japanese researchers from the University of Tokyo
were camped out in the soupy mosquito-rich marsh of Lynn Lake,
Manitoba. This far north, they knew, the earth's magnetic field would
kindly direct travel-worn charge particles from galaxies unknown into the
welcoming arms of their balloon gondola. The Japanese have come in
search of antimatter, an annual pilgrimage since 1993. They plan to do
the same in 2000.
The 1999 balloon launch went off without a hitch -- and without a sound.
The silence, in fact, was as magnificent as the deafening power of the
'Chandra X-ray Observatory's Cape Canaveral shuttle launch just weeks
before. Now_ NASA's largest balloon, 60 storeys high and 40 million cubic
feet in volume, was lifting a 2.5-ton antimatter collector to a height of 37
kilometers. The launch seemed almost feminine compared to the
testosterone-and-fuel-driven rocket launches for which NASA is known.
But the Japanese team wasn't interested in poetics, just good science.
"The discovery of anti-helium would be stunning," said Shuji Orito, who
leads the project known as BESS (Balloon-borne Experiment with a
Superconducting Solenoidal magnet). Just one little anti-helium particle
in their bag of anti-protons and other cosmic rays would point to the
existence of entire antimatter galaxies. "Low-energy charged particles
such as anti-helium are incident only into the polar regions," said Orito,
",,,,'here balloon,; can reach." but where satellite or space station orbits
C a n n o t.
Orito hasn't found any anti-helium yet. "Ne,,ertheless,' said Jonathan
Ormes, a NASA astrophysicist who analyzes BESS results, "we are very
excited every year when we check the latest data hoping to find the first
'Ambassador from the Anti-World.'"
To push or to pull, that is the question. And scientists are increasingly
opting for the latter. Compared to a rocket launch, the balloon platform
is a fraction of the cost; the cargo can be much heavier and unwieldy; and
the instruments can be retrieved, tinkered with, and sent up again.
Tinkering, in fact, has been key to BESS' durability. The first year only
harvested a few antiprotons; subsequent adjustments have brought the
catch to nearly a thousand.
A balloon payload also doesn't require the lengthy and rigorous testing
and the expensive shock padding that a rocket launch does. "The first
thing you do to a delicate satellite when you launch it on a rocket is shake
the hell out of it," said Jack Tueller, project scientist for NASA's Ultra-
Long Duration Balloon project. "This is like putting it on the back of a
truck and driving around the world a couple of times," With a balloon
launch, he said, there is virtually no shaking, and the instrument doesn't
need to be folded into the shroud of a rocket. "If something does go
wrong," Tueller said, "you can retrieve the instrument and launch it
again."
For Tueller, the balloon is not just the poor man's rocket. He said that for
a variety of science, the balloon platform is simply more practical.
Tueller has even come up with an interesting pound-per-day comparison
to 'rocket launches. A Pegasus rocket could carry 500 pounds of
equipment for tWO. years in space;, a balloon could soon carry 3,000
pounds for 80 days. That's on the same scale. The cost of a balloon
platform, however, is staggeringly lower. The 400-kilometer jump from a
high-altitude balloon to a stable low-earth orbit packs on millions of
dollars in extra costs and months, if not years, of development planning.
Thus, lower cost and flexible launches make the balloon platform the
perfect test bed for new technologies. Elbow deep in balloon projects,
Tueller also leads the effort for InFOCIaS, an instrument that will provide
for the first time focusing optics for hard x-rays. This telescope will have
an 8-meter focal length with a new type of "gamma-ray CCD" made from
CdZnTe crystal, providing up to 30-arcsec angular resolution for X-rays at
65-80 keV. There are fewer pixels than what is on a CCD (only 64x64),
but the energies are greater than optical and soft X-ray photons. Such
technology is a key component of NASA's Constellation-X satellite, way
down the pike in 2008. InFOCbtS will fly on a balloon in June, 2000, and
will fly many times after that to work out the kinks. Balloons, Tueller
said, fly current technologieg today.
Bread and Butter Ballooning
The bread and butter of scientific ballooning these days is essentially the
same as it was a hundred years ago: cosmic-ray, gamma-ray and
atmospheric studies. The atmosphere shields much of the universe's
high-energy radiation and wandering particles from reaching the Earth's
surface (see chart), so scientists heed to go up into the stratosphere or
space to collect them.
Cosmic rays, chunks of atoms that bombard the earth at nearly light
speed, are as mysterious now as they were when German physicist Viktor
Hess first discovered them on his precarious balloon flights in 1912.
Some of these atomic particles, at 1020 electron volts, carry nearly as
much kinetic energy as a major league fastball. Where do these particles
come from, and what histories can they share?
One cosmic ray historian is the Isotope Magnet Experiment, or ISOMAX, an
instrument that measures beryllium-10 much like an archaeologist
measures carbon-14. Radioactive 10Be has a half-life of 1.5 million years
and decays into the more stable 9Be. The ratio between the two,
'iherefore, can reveal how long it takes for cosmic rays to reach the earth.
Also, beryllium is solely produced by fragmentation of heavier elements,
created by collisions in the interstellar medium, and serves as a tracer of
the matter it has traversed. At two tons, though, ISOMAX would cost up
to $100 million to launch into space, requiring a Space Shuttle
deployment. The ISOMAX superconducting 10,000-gauss magnet alone is
over a thousand pounds. (They store ISOMAX in its own isolated shed
with a red light warning those with pacemakers to stay clear.) The
balloon launch costs only about $200,000, about a quarter of the annual
ISOMAX budget. ISOMAX is set to fly again in August; by autumn, the
team hopes to announce the long-awaited results from the first flight in
1998.
Atmospheric experiments fly 10 times higher than weather balloons.
Geoffrey Toon (JPL) used a balloon last year to help prove that the ozone
l_,_s observed each summer over the North Pole is a natural phenomenon.
His team flies the 1,800-pound JPL MkIV experiment at 39 kilometers for
about a day at a time. Toon found enhanced amounts of stratospheric
nitric oxide (NO) in the Arctic summer, which eats away ozone. The
culprit is the 24-hour sunlight, "which prevents the normal nighttime
conversion of NO and NO: into ozone-friendly N:Os.
New Observations of Ancient Radiation
Some of the more exciting balloon experiments now being flown concern
the cosmic microwave background (CMB) radiation. The CMB is the
remnant heat from the Big Bang, which has cooled down considerably
since that initial spark to a chilly 2.728 degrees above absolute zero.
Observations of the CMB take us back to about 300,000 years after the
Big Bang, billions of years earlier than any of the light that Hubble can
detect. More importantly, accurate measurement of the CMB yield the
value of f2, the total density of all matter in the universe.
In 1990, the Cosmic Background Explorer (COBE) became the first satellite
to nail down temperature fluctuations in the CMB. Such fluctuations,
however slight, point to density differences in the tiny ball of the newly
expanding universe that later gave way to galaxies. NASA's successor
satellite to COBE is the Microwave Anisotropy Probe (MAP), which will be
launched in 2001 and followed by the even more sensitive ESA Planck
mission in 2007. Cosmologists just couldn't wait, though. Over the past
few years, they turned to balloon experiments that carried detectors up to
100 times more sensitive than those flown on COBE.
BOOMERANG, a clever acronym for Balloon Observations of Mitlimetric
Extragalactic Radiation ANd Geophysics, is an international effort led by
Andrew Lange at Caltech. The experiment flies over Antarctica every
January. This can be more expensive and riskier than North American
flights, due to the location and tricky winds. Yet the constant daylight and
lack of day-night temperature fluctuations allow" the pressurized balloon
to stay afloat longer to collect more data. The January 1999 effort was a
textbook flight, with the balloon traveling 8,000 kilometers around the
pole and landin_ only 50 kilometers from the launch site 10 1/2 days
later.
The BOOMERANG results, released in April 2000, made headlines. The
data provide the clearest image yet of the early universe, an era when the
first atoms formed, allowing light from the big bang that had been
absorbed in a soup of subatomic particles to finally shine through. The
data also support the notion that we live in a fiat, Euclidean Universe that
will expand forever, Lange said. BOOMERANG will fly once again in
January 2001, this time looking for polarization, as opposed to
temperature fluctuation, which some theorist predict could trace the
gravitational wave background from the inflationary period in the
moments that followed the Big Bang. The team is still waist-high in data
from this year's flight, with enough number-crunching to keep a
supercomputer busy for several months straight.
Paul Richards (UC Berkeley) leads the MAXIMA (MAX Imaging Array)
experiment, which has made two overnight flights above Palestine, Texas,
and will fly again in October 2000. He said his team is also overloaded
with data analysis. Whereas about 10 months of COBE observations
captured the whole sky in 5,000 pixels, three hours worth of MAXIMA
balloon data paints a 30,000-pixel picture. MAXIMA finds that the
universe is filled with 5 percent ordinary matter; the remainder is either
cold dark matter, the unseen mass that holds galaxies, or dark energy, a
mystifying repulsive force that seems to be accelerating the expansion of
the universe. The results released in May show spectacular agreement to
BOOMERANG, a good confirmation that we are getting the science right,
Richards said.
NASA's TopHat and the international BAM (Balloon Anisotropy
Measurement) experiment, two other efforts to map the CMB, did not in
1999 as scheduled for technical reasons. They are now waiting a full year
'to catch the right winds for their particular type of balloon flight -- a blow
to these teams racing to collectprime data before the MAP launch. The
yearly delay due to wind cycles is one downside to the ordinarily flexible
balloon launch platform. All is certainly not lost, however, according to
BAM leader Mark Halpern (University of British Columbia). He said no
balloon can truly compete with MAP, but balloon-based CMB data,
regardless of when it is collected, is an inexpensive and invaluable
crosscheck to MAP data.
It's a Bird! It's a Plane! It's a Telescope!
Although the atmosphere kindly allows visible light to penetrate,
astronomers cannot help but take advantage of clearing 99% of the air
above us. Stratoscope I and II, flown in the 1950s and 1960s by the late
Martin Schwarzschild (Princeton University), were essentially precursors
to the Hubble Space Telescope. The first, a 30-cm solar telescope,
obtained extraordinarily sharp pictures of the corona; its successor, a 90-
cm mirror lofted at heights of over 32 kilometers, made several
photographs of planets and star systems with a resolution close to a tenth
of an arc-second. Some astronomers today are striving for millisecond-
arc optical imaging, an equivalent of a 10-meter ground-based telescope
without the need for corrective optics voodoo, which could outdo Hubble.
, • S
The French PRONAOS (Le Programme National d observatton
Submillimetriques) obtains up to 5-arcsec resolution in the infrared with
its 2-meter wide telescope. At 40 kilometers high, PRONAOS clears the
water molecules in the atmosphere that make a mess out of ground-based
infrared observations. In this regard; PRONAOS is a precursor to the
Space InfraRed Telescope Facility, an orbiting 85-cm telescope and the
final installment of NASA's "Great Observatories."
The Solar Disk Sextant and the Flare Genesis Experiment are two of many
sun chasers. Flare Genesis is an 80-cm aperture telescope that measures
the sun surface magnetic fields and motions. With 10-day flights over
Antarctica, the experiment is searching for the solar conditions that lead
to solar flair. The telescope provides 0.2-arcsec resolution, compared to
SOHO's 4-arcsec resolution. Solar Disk Sextant has gone far in accurately
determining of the shape of the sun. The experiment now' hopes to
graduate to Earth orbit, however, because temperature changes from the
ground through the troposphere and stratosphere limit the instrument's
seasttlvlty.
Inflatable Sandwich Bags
[t is a little known fact that the United States mainland was bombed
during WWII. In retaliation to the Allied-led Doolittle Raid on Tokyo on
April 18, 1942, Japan spent the next two years constructing 9,000
balloons of paper and silk, for rubber was too precious to sacrifice.
Women and children were recruited to turn mountain root vegetables into
glue for the balloon fabric. During a five-month "bombing" campaign
startin_ on Nov. 3 1944, Japan launched these backyard balloons with
real anti-personnel and incendiary bombs into the high altitude winds that
would carry them more than 9,500 kilometers eastward across the Pacific.
Nearly a thousand reached the States, some setting off forest fires. On
May 5, 1945, six picnickers were killed in Oregon when a balloon bomb
they dragged from the woods' exploded. These were the only known
fatalities occurring within the United States during WWlI as a direct result
of enemy action.
Paper and silk were the fabrics that carried countless balloon adventurers
to altitudes of 9,000 meters for more than 150 years. By the early 20th
century, rubber and new synthetic fabrics being developed at Dow
Chemicals were lifting balloons higher and higher. Today, fabrics about
the thickness of plastic sandwich bags carry 3,000-pound loads to the
fringes of space.
The NASA National Scientific Balloon Facility in Palestine, Texas, is behind
most of the world's high-altitude balloon launches. From the Antarctic to
the Canadian flatlands and a few deserts in between, the NSBF sets afloat
about 30 balloons a year to heights of 30 to 55 kilometers. Balloons
come in five classes, ranging in size from I1 to 40 million cubic feet, the
largest roughly 170 meters in diameter when fully inflated (large enough
to hold fleet of Boeing 747s).
Balloon launches usually involve a crane and a dozen or so personnel.
The crane keeps the payload a few feet off the ground for the launch. The
balloon remains stretched out along a runway. As the balloon slowly
inflates with helium, it rises above the payload. The crane follows the
balloon as it passes over the payload, keeping the payload from dragging
'filong the ground. As the balloon begins to lift the payload, the crane lets
go. The balloon will climb for-several kilometers before fully inflating.
High into the clouds, what looks like a long, thick rope connecting the
balloon to the payload is actually uninflated balloon material.
The launch can be exhilarating, yet most scientists will tell you the wait
can be fantastically boring. Balloonists are at the mercy of the wind and
the rain. A launch could go off like clockwork, or it might be postponed
day-by-day for two weeks. Hacky-sack and pinochle only whittle away the
time so far in the 50 degree Celsius heat of Australia's Alice Springs.
, :g.-g
Countdown to the 100-Day Balloon
Steve Smith and Henr,, Cathey are busy these days blowing up balloons
and popping them. Thev lead a team of engineers at Wallops Flight Facility
constructing 100-day balloons, the essential goal of the Ultra-Long
Duration Balloon project. An extended balloon platform opens up endless
possibilities, such as a floating Keck or even communication towers. The
first ULDB flight with real cargo (a cosmic ray experiment called TIGER) is
set for December 2001.
Currently, high-altitude balloon flights last only a couple of days. The
main problem is temperature fluctuations from day to night. The sun hits
the balloon; gas warms up and expands; and the excess expanding gas is
vented out to the atmosphere. Sun sets; gas cools; the gas contracts;
balloon volume decreases; and the balloon sinks. Balloonists can buy a
day to two by dropping ballast at night. They can fly up to 22 days in
Antarctica, where summer days (or nights) last half the year and there is
no day/night temperature fluctuation. But eventually the balloon comes
down.
The ULDB is a sealed, super-pressure vessel that can maintain pressure
differences at day and night. As such, Smith's team needed to find a
tough new material and design that could resist pops and tears and hold
up to the prolonged UV exposure in high altitudes. The ULDB material --
about the thickness of a sandwich bag at 0.0381 mm -- is an extruded
polyethylene film with a high-density layer in between two linear low-
density layers, "like a slice of salami on soft white bread," said Cathey.
The "salami" part provides strength; the "bread" provides surface
toughness and enables a secure seal between the sheets of film that form
the balloon envelope. The unique pumpkin-shape design shifts most of
the stress to the tendons. This has allowed engineers to use a lighter,
more durable material than what is used on spherical balloons to support
3,500-pound payloads.
Smith's main worry is trajectory control, the ability to prevent the balloon
from drifting over unfriendly countries or controlling its direction in
.general to maximize the science of a particular experiment. "Trajectory
control is the biggest technological leap for the ULDB," said Smith, the
ULDB project manager. "It could make or break things for flying only 60
days or for 100 days." Another concern is fine pointing for arc-second
imaging, for balloons at that height tend to rotate. Such issues will likely
be worked out, Smith said. in the first couple of years following the test
Ilight.
Spying on Extra-Solar Earth-Sized Planets
NASA's Olympus Study Group, named after the legendary mountains on
both Earth and Mars, identifPes ambitious scientific missions and the
corresponding technology in both earth and space science that would
benefit from this long duration balloon platform, according to group
leader Robin Mauk, a systems engineer at Goddard. She said that early
proposals to the ULDB project include extra-solar planet searches and the
first hard x-ray surveys of the entire sky. A promising commercial
application is station keeping, essentially floating cell phone towers and
other telecommunications bases that currently blight the landscape.
Holland Ford (Johns Hopkins University) and Larry Petro (Space Telescope
Science Institute) hope to fly a 2.5-meter optical telescope and
coronagraph at a height of around 36 kilometers to search for Jupiters
and perhaps Earths around the brightest stars within 10 parsecs. Their
instruments could provide four orders of magnitude improvement over
HST WFPC-2 and the biggest ground-based observatories. Harvard's John
Grindlay envisions southern and northern balloon launches for an X-ray
telescope called EXIST-LITE, which would survey the 20-600 keV X-ray
band with 10 times the sensitivity of the upcoming INTEGRAL mission.
Another missions slated for the ULDB is SOAR, a far-infrared telescope
with diffraction-limited resolution over a field view of 6.25 arcmin -- a
precursor to a 3-meter balloon-borne telescope. Led by Giovanni Fazio
and Gary Melnick (SAO), the goal of SOAR is to detail the birth and
evolution of galaxies, as well as stars in our own galaxy.
ii
Balloons Over Mars
Plans for extraterrestrial balloons are also afoot. Mercury and Pluto are
the only planets that do not have an atmosphere to support balloon
flights. The rest, as well as a few moons, are fair game. In 1985, the
Soviets floated two balloons over Venus for 48 hours, gathering data on
atmospheric temperature, pressure and winds. NASA and ESA are now
considering plans to send balloons to Venus, Mars and Saturn's Titan.
Ideas include collecting samples of the new worlds with drag ropes or
probes and letting balloons wander with cameras snapping images of the
surface with resolution 10,000 times greater than satellite shots.